Not sure I exactly understand what you mean by high velocity...but whenever you increase velocity into a system (duct)that is maybe not adequate for the volume of air....what you are likely to get is a higher level of noise through each vent. Depending on the amount of increased velocity, will depend on the amount of increased noise level. It may effect comfort but this depends on amount of velocity increase as well.
It's a common myth that SpacePak's high velocity heating and AC system is going to be louder than a conventional system. Part of the reason is that few people understand that a typical high velocity system runs on a little more than half of the CFM of a conventional ducted system. You are correct in assuming that moving a large volume of air through smaller ducts will cause noise. SpacePak runs roughly 220-250 CFM per ton of cooling vs the usual 400. The air is distributed through the duct at a much higher static pressure (1.5" wc) in order to achieve better distribution. Proper cooling is achieved through the use of a thicker (6 row) coil that produces a deeper delta T (28-30 degrees F vs conventional at 18-20 degrees F). They also employ the use of 3' sound attenuator tubes at the end of each 2" supply hose run to capture a good deal of the air noise. Bottom line...if it's installed properly, high velocity can be just as quiet, or quieter than a conventional ducted system.
It sounds like the original poster has an ideal application for retro-fitting high velocity. The lack of space for ductwork makes a perferct argument. SpacePak is sold as a complete system, which includes the air handler (which has a more powerful motor to create the higher static pressure and the above mentioned 6 row coil), ductwork and return duct. A qualified HVAC contractor is recommended to size, lay out and install the system, as well as install the outdoor compressor and line set which are components not supplied by SpacePak.
SpacePak can supply both heating and cooling with no problems. Heating options include a hot water coil, heat pump, or modulating electric strip heat. The winter design temperatures for your area will help decide which is best.
If you tell me where you are located, I can recommend someone to call for help.